Technical considerations in aortocoronary bypass grafting

Biomed& Pharmacodter(1990)44.359-364

359

BElseuier.Paris

AR Dresdale,

G Paone,

NA Silverman

Divisiim of &?di@c and Th~racic Surgery. Henry Ford Hospiful. 2799 West Grand Boulevard, Dead,

Mf 48202, USA

(Received 22 June 1990; accepted 2 July 1990)

Summary - Since the inception of aortocoronary bypass surgery, many technical advances have been rapidly achieved. Early experience was limited to reversed saphenous vein grafting of single vessel coronary artery disease. M~tipie grafts to several vessels soon became commonplace and sequential grafting techniques were developed. Expanded use of the internal mammary artery resulted after analysis of superior patency rates achieved with this conduit. Use of alternative conduits such as upper ext~ity veins, allogenic veins, synthetic graft material ~~lyte~~~om~y~ene~, radial artery,splenic artery, and gastroepiploic artery have been explored. Apart from the gastroepiploic vessel, none of these alternative conduits have been suitable. A grat deal of effort has been directed at the mechanisms of saphenous vein occlusion including technical considerations, early thrombosis, intimal hyperplasia and graft atherosclerosis. Platelet inhibition and lipid reduction have shown promise in improvjng patency rates. Further work in these areas should lead to even better results.

coronary bypass surgery I graft patency f bypass conduits

R&urn&- Les implications techniques des greffes coronariennes.Depuis I’intruduction de la chirurgie du pontage uorto-coronaire, des progrk techniques nombreux onr vite &k r&lisks. Les premit?res exptkiences se limiraient d: la greffe d’une wine saphBne inverske dans le cas de ma&&e coronarienne ne rouchunt qu’un vaisseau. Les greffes multiples sur plusieurs hissers sent bier&&tever chases b~&~es et i’on a d&elopp& des fechn~ques de greffes s~que~ielles. I’&lis&on de l’wt&e mumnkre interrnes’est r~pan~e tqw& qu’une analyse des r&ultats ait mot& des taut de r&ussite sap&?ieursavec cette m&ho&. L’urilisation de m&odes alternutives telles la greffe de veines des extrtimiltis supkieures, de vekes ~liog~niq~s~ de ma&iel synt~tiq~ ~po~t~tr~~uor~thyl~ne~,art*?e radiate, art&e spl&ique et arrPre gastro&piploique, a &! exp&ime&e. A l’exception de l’usage du vaisseuu gastro-LpipioFquetaucune de ces m&ho&s alternatives ne s’est montrd adhptie. On s’est attaque’ avec beaucoup d’efforts aux me’canismes de I’occlusion de la veine saphane parmi lesquels les dtifauts de techniques, la thrombose prkose, l’hyperplasie de I’intima et l’arhdrosclkrose du greffon. ~inhibit~n des plaquertes et la reduction des iipides se sent montrkes porteuses de promesses dnns I’am
Contemporary aortocoronary bypass surgery has reached a level of sophistication and success that could not have been foreseen in 1962 when David Sabiston performed the first coronary artery graft in a human being 1611. Without the circulatory support of cardiopulmon~ bypass, a reverse saphenous vein was grafted end-to-end to the transected right corona artery after the vein graft had been anastomosed to the ascending aorta. The operation went well technically, but the

patient suffered a cerebral vascular accident and died 3 days later. Post-mortem examination discclosed a thrombus at the saphenous vein to aortic anastomosis and it was concluded that embolization from this thrombus had caused the fatal stroke. Favaloro et al used a segment of reverse saphenous vein graft to replace a stenotic segment of the main right coronary artery [22]. Within a short time he began to use the saphenous vein as

a

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true bypass graft with its origin from the ascending aorta and the distal end anastomosed in an end-to-side fashion to the coronary artixj beyond the obstruction: ~niti~ly~use of the procedure Wtx3 gin&& to patients witb single vessel disease affectiug the proximal aspect of the right coronary artery; it was soon apparent that a similar teehnique could be utilized for lesions on the left side of the heart and the operation was extended to the anterior descending and circumflex arteries [29, 56, 693. In X968, Johnson and Lepley began what was to become an extensive exPerience with coronary artery bypass including tbe use of multiple grafts. Their report in 1970 fJ T&x~c Ga~~~va~c Swg 59,128138) marked the beginning of the modern era of coronary artery surgery E171. Paralleling the early use of reverse saphenous vein grafts, the internal mammary artery (IMA) was also employed as a conduit [28,54]]. Preliminary data suggested that the IMA provided less flow than vein grafts. This impression, combined with the greater technical demands and longer period of IMA surgery, led to its falling into disfavor. Meanwhile, rapid technical advances enhanced the execution of aortocoronary bypass. These advances included the adoption of the near universal use of ~ar~ople8i~ solutions, By arresting and preserving the heart, ~~ioplegia provides a quiet, bloodless operative field and reduces, but does not eliminate, the time pressure placed on the surgeon. magnifying telescopes, which markedly improve the quality of microanastomoses, as well as refined instruments and sutures, have all contributed to improved technique. Over the past 5 years, scrutiny vf early data has made it clear that IMA grafts d~atically ou~e~o~ reverse saphenous vein grafts. Late Patency- freedom from new coronary events, and long-term survival are all superior when the ZMA is used 123, 26, 441. Since most patients require multiple bypasses9 reverse saphenous vein grafts continue to be the most frequently used conduits. Approximately 10% of saphenous vein grafts occlude in the early postoperative period and in one year this number reaches 15% [30]. Five years after surgery, about 25% of vein grafts are lost. After 10 years the occlusion rate increases to approximately 50% and of those r~aining patent about half will have evidence of sig~~~ant luminal ~a~owing [12-16, 30, 41, 421.

The patency rate for IMA grafts on the other hand, is greater than 90% after 10 years 123, 26, 27, 30, 441, interestingly, the IMA may also be used as a free graft apparently without sacrificing its superior patency rate [46]. If it is necessary to obtain additional length for a very distal anastomosis or to avoid crossing the midline fa patent graft inczreaaes the risk of opening the sternum during a redo operation), the IMA can be detached proximally and distally. Following performancs of the distal anastomosis, the proximal end of the IMA is connected end-to-side, either to the aorta or a vein graft. In light of the superior patency of single IMA grafts, some surgeons have a~emPted to increase the number of vessels bypa~ed with an IMA by using sequential ~astomoses. though published data is favorable, application of this sequential technique is limited by both technical and anatomic constraints [37, 48, 661. optimization of event-free and long-term survival may be achieved by using both internal mammary arteries f23, 271. Evidence suggests that the use of bilateral IMA grafts increases SW gical morbidity related to bleeding, wound healing and myocardial protection. The consensus however, appears to be that tbe increased risk is most likely limited to media~~itis in elderly diabetic patients f39, 455 Alternative conduits have been used for eoro= nary artery bypass, but usually with very poor results. Upper extremity veins do poorly as arterial grafts [64]; probably less than 10% are patent after 6 years. Allogenic veins, either fresh or cryopreserved, also result in early occlusion. Synthetic in pa~ic~l~ conduits, expanded polytetraflu~roethylene (PTFE), have been studied as aortocoronary grafts, but long-term patency is too low to justify routine use 124, 55j, In addition to the WA, various other autologous arteries have been used as bypass grafts. Radial artery grafts have proved unsatisfactory ia the short-term because of their tendency to spasm and in the longer term because of concentric intimai hy~~lasi~ which causes a graft occlusion rate greater than SO% at 6 months [24]. Splenic artery grafts have also been attempted and subsequently discarded because of technical dif~c~lty in procurement and a high incidence of atherosderosis WI. Of the alte~ative arterial conduits to the IMA, only the gastroepiploi~ artery? based on preliminary evidence, has acceptable early patency rates

Technical considerationsin aortocoronarybypass grafting 147, 53, 58, 651. Histological examination of this vessel, however, reveals an ominous finding. Unlike the IMA, in which the vasa vasorum runs only in the adventitia without penetrating the media, the gastroepiploic artery resembles the radial artery with a well developed vasa vasorum in the media [23, 241. It is theorized that vessels with this configuration depend on an intact vasa vasorum for blood supply to the outer 2/3 of their walls. The IMA differs not only from the radial and gastroepiploic arteries, but also from the basilic and cephalic arm veins, by deriving most of its nutrient blood supply form luminal diffusion. This anatomic and physiologic difference might explain the excellent patency of free IMA grafts and raise scepticism about the long-term patency of free gastroepiploic grafts. When vessels with well developed vasa vasorum are detached and used as free grafts, their source of nutrient blood is disrupted. Hypoxic and without adequate nutrients, these vessels are vulnerable to cell death, fibrosis, and ultimately graft occlusion. Multiple histological changes occur in reverse saphenous vein grafts [l, 3-6, 11, 19, 25, 33, 38, 41, 42, 60, 63, 67, 681. These changes correlate with our understanding of the physiological events leading to graft stenosis and occlusion. The vein endothelium disappears after grafting into the arterial circulation. The denuded endothelium exposes the subintimal layer to platelets. Uninhibited by pharmacologic intervention, platelets tend to clump and promote medial cell migration and proliferation to the intimal layer. Secretion of growth factor is thought to mediate this cellular activity. The intimal thickening which occurs may be a precursor to atherosclerosis. Reverse saphenous vein graft endothelial cells may also decrease production of prostaglandin and lose their ability to clear lipids and fibrin 19, 13, 35, 50, 51, 601. Early graft failure may result from technical errors in constructing the anastomoses. In addition, excessive or inadequate graft length may result in kinks or increased tension, either of which can cause early graft failure 129, 34, 691. The patency rate is also dependent on graft flow and distal vessel runoff [19, 211. After the first year however, these factors do not affect patency. Many technical modifications have been advanced to decrease the early graft failure rate [2, 7, 321. At the time of vein harvesting, much attention is given to avoiding excessive distention because it may disrupt the intima and cause me-

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dial injury 12, 12, 31, 32, 41, 591. As well as improving technique, attention has been focused on the composition of the irrigating solution [I, 30, 32, 40, 43, 57, 601. Platelet inhibitors have also been studied. Experts have demonstrated early replacement of endothelium by a platelet layer. Contact with endothelium stimulates platelets to release a potent vasoconshictor thromboxane, as well as a growth factor that stimulates medial muscle cells to proliferate and migrate to the intima. Experimental work with platelet inhibitors has demonstrated a beneficial effect on reduction of both thrombosis and intimal hyperplasia [20, 36, 38, 49, 521. Several clinical trials using aspirin and dipyridamole in humans have shown improved early graft patency following aortocoronary bypass [lS, 161. Aspirin blocks thromboxane A2 stimulation of platelet aggregation by inhibiting cyclooxygenase. Dipyridamole inhibits phosphodiesterase thereby increasing cyclic AMP and potentiating aspirin’s inhibition of platelets [62]. Until more data becomes available over a longer period of time, we can only speculate whether the atherosclerotic process in reverse saphenous vein grafts will be retarded by the use of these agents. Their efficacy in maintaining patency of arterial conduits has not, however, been defined. A high risk lipid profile (high LDL, low HDL) is associated with graft atherosclerosis at 10 years. More studies are required to investigate the potential benefits of diet and pharmacologic reduction of blocd lipids to improve long-term vein graft patency. One recent study did suggest a beneficial effect of such pharmacologic intervention [8]. Coronary artery bypass surgery has shown to be an effective modality for restoring myocardial blood flow, preserving ventricular function, improving the quality of life and prolonging life expectancy in selected subsets of patients 1621. As the search for the ideal conduit continues, expanded use of arterial grafts and pharmacologic manipulation to decrease closure of vein grafts may help to decrease the need for reoperation.

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